Charged particle detectors, and, in particular, electron detectors are critical for high-contrast image formation from secondary electrons in SEMs. Conventional detectors used to detect secondary electrons have large dimensions. Miniature electron beam (E-beam) columns have small, closely spaced apertures and lens components, and all components are in close proximity to the sample. These characteristics make it difficult to mount conventional secondary electron detectors in miniature E-beam columns and achieve the high collection efficiencies required for good signal-to-noise ratios.
An Everhart-Thornley detector (ETD) is a device commonly used for collecting secondary and backscattered electrons in SEMs. An ETD comprises a biased collector grid surrounding a scintillator material, which is coupled to a photomultiplier to provide a first stage of amplification. ETDs can be mounted in SEMs as “in-lens” detectors, situated within the column above a pole piece. This type of detector configuration can be used to detect electrons that are ejected from a sample and drift back up the column. The ETD is also commonly positioned beneath the pole piece in proximity to the sample to detect both backscattered electrons and secondary electrons.
However, the large size of the ETD makes it impractical for “in-lens” mounting in a miniature electron beam column because the lenses are typically separated by 0.1-10 mm, which is small compared to the dimensions of the ETD detector. Mounting the ETD below the pole piece restricts the working distance. ETDs are further limited by their small solid angle for collecting incident electrons. This results in relatively poor collection efficiency, and thus inferior signal-to-noise ratios for a given beam current.
Micro-channel plate (MCP) detectors are also used for detecting secondary and backscattered electrons in SEMs. MCPs are constructed of a grid of channels, typically 1-100 um in diameter, and often oriented at a slight angle to the incident beam. When a voltage is applied between the top and bottom of channel plates, incident electrons are accelerated and multiplied, resulting in current gain. The thickness of the MCP is typically 0.4 mm or greater to achieve sufficient gain. In fact, for high-contrast imaging, MCPs are often stacked together in a dual chevron configuration to increase the overall detector gain. However, when packaged using conventional techniques, a dual stack limits the SEM's working distance.
Accordingly, a new apparatus and method are needed for detecting radiation and charged particles in a miniature SEM.